Fracture toughness evaluation in C(T) specimens with reduced out-of-plane constraint

During fracture toughness testing of C(T) specimens, an important assumption is that the test specimen is highly constrained. This is ensured by testing a deeply cracked specimen, with in-plane and out-of-plane dimensions that are sufficient to guarantee an appropriate level of crack tip stress triaxiality. This condition guarantees that high-constraint fracture toughness values are derived, conservative for use in standard fracture mechanics assessments. In reality, many components have small out-of-plane dimensions (small thicknesses). This often causes a reduction in crack tip constraint of a sufficient amount to increase the effective fracture toughness of the components. However, there is currently limited understanding as to the magnitude of the benefits that could be claimed from out-of-plane constraint loss.Finite element and damage modelling of thin C(T) specimens under different loading conditions has been undertaken, looking at the effects of loss of out-of-plane constraint, to help validate the results of an on-going testing programme. When available, data from testing of thin C(T) specimens could allow the parameters of the damage model, based upon a ductile criterion, to be calibrated. Material resistance to fracture under different situations has been determined, leading to a correlation of toughness to the constraint condition for a nominal set of material parameters.